The invention broadly relates to a process for treating glass in order to vary the physical, optical and chemical properties of the glass and more particularly to a process for applying a metal to a glass wherein said metal is incorporated therein without any residue remaining on the surface of said glass. The metal reacts in such a way with the glass so that the composition of the glass is changed in the region where this metal is incorporated, thereby resulting in a change in the properties of the glass which differ from those of the glass prior to treatment.
Various processes are known in the prior art for changing properties of a glass, such as the strength of a glass, for example, which can be accomplished by an ion-exchange process whereby metal ions are allowed to diffuse into a glass. This diffusion process takes place at elevated temperatures, usually in the range of the transformation temperature which is either just below this temperature (no less than 100.degree. C) or above this temperature (no greater than 300.degree. C). In addition to the strength of the glass, other properties of the initial glass can be modified in the diffusion range, including the surface tension, linear thermal expansion, electrical resistivity, the refractive index for various light wavelengths, dispersion, light permeability, chemical resistivity to acids, caustic solutions and other liquids, the tendency to crystallisation, the relaxation properties and the dependence of viscosity on the temperature.
In all of these prior art processes, the materials containing cations of the components to be used for the diffusion of desired components, whether used either as a salt melt, or as a solid mixture, such as a dried clay-metal salt suspension, are brought into contact with the surface of the glass. The cations that are present in an ionic bond tend to diffuse into the interior of the glass.
The prior art ion exchange processes are characterized by the exchange of cations; i.e., with ion exchange processes, cations emerge from the glass and other cations originating from a medium surrounding the glass migrate into the glass and occupy the positions of the ions which have migrated from the glass.
A significant disadvantage of the ion-exchange process is that it is not possible to modify the properties of glasses in very small regions because the simplest and least expensive process involving the dipping of the desired portion of the glass into salt melts for the ion exchange, does not permit the modification of only small regions of the glass portion when contacted with the ions, e.g., sodium ions or potassium ions.
Consequently, a need exists to find a process for treating glass surfaces in a simple and efficient manner and in a particularly defined region whereby the properties as a whole or in part can be modified by any subsequent or simultaneous simple processes. Available as such simple, simultaneous or subsequent processes are tempering processes (temperature conditions defined as a function of time).